Mask Tips:
The great thing about masks is that you can paint on black to block areas of the layer and white to reveal. Just keep one hand on the "x" key, and you can easily switch between black and white (or whatever foreground, background color you have chosen -- hit "d" for the default colors: black and white). If you're masking, say, the background out of a photo of a person, this makes mistakes easy to correct.
Also try: using gradients on a mask or painting in shades of gray onto the mask.